In WLAN deployments without mesh services, client stations or end stations (STAs) must associate with an access point (AP) in order to gain access to the network. These STAs are dependent on the AP with which they are associated to communicate.
A so-called mesh network appears functionally equivalent to a broadcast Ethernet from the perspective of other networks and higher layer protocols. As an example, the mesh network may be a LAN according to the IEEE (Institute of Electrical and Electronics Engineers) 802.11 specifications, where links and control elements forward frames among the network members. Thus, it normally appears as if all mesh points (MPs) in a mesh are directly connected at the link layer. This functionality is transparent to higher layer protocols.
A standard ‘infrastructure’ wireless Local Area Network is a centralised network in which STAs attach to the AP which acts as a ‘master station’. This centralised topology makes network formation and initial channel selection easy. The AP is configured to start transmitting at a certain frequency channel and the STAs only need to find this channel e.g. by scanning a list of available frequencies. They can do so actively, by broadcasting probe requests on each visited channel, or passively, by listening for advertisements or beacons on each visited channel. After having visited all available channels, they will have found all APs that are in the vicinity, and can select one to associate with.
In a wireless mesh network, however, there is no central coordinating station. This makes network formation and initial channel selection more difficult than it is in a standard infrastructure network. In particular, problems exist in that a STA that performs active scanning might find a number of other stations on various frequency channels. In this case, it will need to choose and the current draft of the relevant standard prescribes a protocol based on channel precedence values for initial mesh formation (cf. Clause 11B.4 of the Draft Standard for Information Technology—Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Amendment: Mesh Networking, IEEE Unapproved draft, IEEE P802.11s/D2.0, March 2008). According to this protocol, each mesh node selects a channel precedence value. Next, it carries out some active and passive scanning. Then, if no suitable neighbors are found, it selects a channel and advertises its channel precedence value. If it finds multiple neighbours, it selects one of these based on the numerically highest channel precedence value. However, this protocol is flawed in two ways.
As a first problem, often disjoint meshes are formed during mesh formation in a number of important scenarios.
FIG. 2A shows a schematic mesh-type environment with four exemplary mesh nodes MPP and MP2 to MP4 during start up. One of the mesh nodes is a mesh point portal MPP which has a peer link with a second mesh node MP2 on a channel “a” and with a channel precedence value (CPV) of “3”. A fourth mesh node MP4 is out of range with the second mesh node MP2 and has stopped the active scanning process and has adopted one of its mesh profiles, a channel “b”, and a CPV of “5”. The CPV of the fourth mesh node MP4 is selected at random and happens to be higher than the CPV advertised by the second mesh node MP2.
FIG. 2B shows the schematic mesh-type environment of FIG. 2A after a disjoint mesh has been formed due to the fact that a third node MP3 has used the above CPV-based protocol selection mechanism to select a candidate peer node. According to the above selection mechanism, the third node MP3 shall select the fourth MP4 with the higher CPV of “5” as the candidate to establish a peer link with, so that two disjoint meshes have been formed, rather than one connected mesh.
As a second problem, the purpose of multi-radio devices is completely disregarded.
FIG. 3 shows a mesh-type environment with a multi-radio device comprising first to third radio devices A to C and two mesh nodes D, E. Assuming that the two mesh nodes D, E select candidate peers on the basis of the CPV, the purpose of multi-radio devices is not considered. In the situation show in FIG. 3, the radio devices A to C have stopped scanning, have selected channels “1”, “3” and “5”, respectively, and advertise respective CPVs of “2”, “7” and “4” for their chosen channels. Any new device or node in the environment that starts up will find the three radios A to C. However, because of the conventional selection mechanism of the above protocol, it has to select the radio that advertises the highest CPV. In FIG. 3, this is the second radio device B, as it advertises a CPV of “7”, whereas the first radio device A advertises a CPV of “2”, and the third radio device C advertises a CPV of “4”. Consequently, the first and third radio devices A, C will be completely ignored during mesh formation and will not establish peer links. I.e., when the two mesh nodes D, E start up, they both will select the second radio device B as candidate peer and ignore the other radio devices A, C.